1. Field of the Invention
The present invention relates a method of controlling a power source of a camera, wherein the power source consists of at least a battery and a booster circuit for boosting a battery voltage up to a predetermined level, and supplies the voltage to respective elements of the camera.
2. Background Arts
Many of recent cameras have electronic mechanisms and devices mounted therein, such as an automatic focusing device, an automatic exposure control device, a motor-driven zooming mechanism, and an electronic shutter. In these cameras, a microcomputer is used for controlling the electronic elements sequentially. Also, a display device, mostly an LCD (liquid crystal display) panel, is often integrated in the camera, for displaying information necessary for photography, like whether a photo filmstrip is loaded or not, the film speed, the number of photographed picture frames or the remaining number of available exposures, the present date and time that is to be superimposed on each picture frame, the present camera mode, the charge condition of power source batteries, and so forth.
As the power source batteries, the cameras generally use a lithium cell, like CR123A, or an AA-type battery, whose voltage is about 3V. To obtain a higher operating voltage, e.g. 5V, enough for the microcomputer and other electric elements to operate, the power source voltage from the battery is boosted up through a booster circuit when a main switch of the camera is turned on to switch the camera from an OFF mode to an ON mode for enabling photographic operations. In those cameras where the microcomputer should operate during the OFF mode, e.g. for operating the clock, or for monitoring the switching condition of the main switch, the microcomputer can operate at the battery voltage.
Because the camera consumes a lot of current while the booster circuit is active, the battery will run down soon if the booster circuit is kept active throughout the ON mode. To avoid such inconvenience, most of the commercially available cameras have a function to deactivate the booster circuit and turn off the power source automatically when the camera has not been operated for a predetermined time while it is set in the ON mode. This function will be called an auto-off function. The auto-off function suppresses the waste of battery, to make the battery last for a longer time.
Indeed the auto-off function deactivates the booster circuit after the camera stays in its standby condition for a certain time, but it does not reduce the consumption of the battery during the standby condition. Accordingly, in order to save the consumption of the electric power effectively, the booster circuit should be turned off as soon as possible while the camera is in the standby condition. However, if the booster circuit is turned off frequently, the operation facility of the camera gets worse.
Japanese Laid-open Patent Application No. 11-38465 discloses another teaching for saving the consumption of electric power, wherein a booster circuit is not activated but a low speed CPU operates at the low battery voltage during the standby condition. The low speed CPU is for operating the clock and monitoring switching condition of a release switch that is turned on by a shutter release operation. Upon the shutter release operation or upon any other operation on the camera, the booster circuit is activated, to boost the battery voltage up to the operating voltage necessary for operating a high speed CPU. The high speed CPU is for controlling sequential operations and data processing. Since the booster circuit does not operate during the standby condition, the power consumption is suppressed, and the battery lasts longer.
However, the latter prior art also has problems. The battery voltage fluctuates largely when a large current flows through the battery, for example, when the respective elements of the camera are activated at once in response to the release switch being turned on, or when a flash circuit is activated. Such a large voltage fluctuation may cause to reset the low speed CPU, putting the clock out of time, or disabling responding to the shutter release operation. Where the LCD is driven at the boosted voltage, the LCD does not display the photographic information in the standby condition, i.e., unless the booster circuit is activated upon some operation on the camera.
Since the LCD may be driven either at the battery voltage of about 3V, or at the boosted voltage of about 5V, it is possible to supply the LCD with the battery voltage while the booster circuit is inactive, and with the operating voltage after the booster circuit gets active. It is also possible to use the battery voltage for driving the LCD without boosting it.
However, as the LCD has a property to vary its display density according to the supplied voltage, the difference between the battery voltage and the boosted voltage results in changing the display density of the LCD so much that the LCD can be mistaken as defective. In addition, if the operating voltage of the LCD is adjusted to one of these two different voltage levels, the performances of the LCD could be remarkably worsened at the other voltage level. On the other hand, where the LCD is always supplied with the battery voltage, the display density varies largely when the battery voltage fluctuates largely each time the respective elements of the camera are concurrently activated in response to the shutter release operation or another operation on the camera.
Furthermore, where the microcomputer functions as the clock for showing the present date and time or dating the photographed pictures, if the battery voltage goes below a lower limit of the operating voltage for the microcomputer in the OFF mode where the booster circuit is off, the clock not only stops clocking, but the date and time of the clock will be initialized. In that case, it is necessary to setup the date and time of the clock again. To avoid such an inconvenience, the battery must be replaced with new one before the battery voltage goes below the lower limit of the operating voltage. This is a waste of the battery.
In view of the foregoing, an object of the present invention is to provide a method of controlling the power source, which is effective for saving the power consumption, while maintaining the LCD in a proper display condition even in the standby condition.
Another object of the present invention is to provide a method of controlling the power source, whereby the clock device of the camera can operate even while the battery voltage is very low, so the battery may be used in the camera as fully as possible.
According to an aspect of the present invention, in a method of controlling a power source of a camera that is provided with a display device, wherein the power source comprises a battery and a booster circuit for boosting a battery voltage up to a predetermined upper level, the booster circuit being turned on and off by a control device, the method comprises steps of:
turning the booster circuit on continuously while the camera is in a drive mode driving photographic mechanisms of the camera, to output voltage of the predetermined upper level continuously during the drive mode;
turning the booster circuit on intermittently at a regular time interval while the camera is in a standby mode waiting for any external operation on the camera, such that the output voltage of the booster circuit varies between the predetermined upper level and a predetermined lower level during the standby mode; and
driving at least the display device by the output voltage of the booster circuit continuously during the drive mode and the standby mode.
Since the booster circuit is intermittently turned on and off during the standby mode, the power consumption is reduced.
By setting the predetermined lower level to be a minimum voltage level necessary for the display device to keep its display density above a predetermined level, the display density is maintained in a satisfactory range during the standby mode, while saving the power consumption.
According to another aspect of the present invention, in a method of controlling a power source of a camera that is provided with a clock device for counting time, wherein the power source comprises a battery and a booster circuit for boosting a battery voltage up to a predetermined upper level, the booster circuit being turned on and off by a control device, the method comprises steps of:
turning the booster circuit on continuously while the camera is in a drive mode driving photographic mechanisms of the camera, to output voltage of the predetermined upper level to necessary portions of the camera;
checking, as the camera moves to an OFF mode where the camera is prohibited from photographing, if the battery voltage is above a reset level below which the clock device of the camera cannot work;
turning the booster circuit off and driving the clock device by the battery voltage during the OFF mode, if the battery voltage is above the reset level; and
turning the booster circuit on intermittently at a regular time interval during the OFF mode, if the battery voltage is below the reset level, to boost the battery voltage up above the reset level and drive the clock device by the output voltage of the booster circuit during the OFF mode.
According to this configuration, if the battery voltage goes below the reset level, the battery voltage is boosted up above the reset level of the clock device to drive the clock device by the output voltage of the booster circuit even in the OFF mode. Therefore, the clock device can continue clocking for a certain period after the battery voltage goes below the reset level.